Internal combustion engines used for both mobile and stationary applications are subject to strict emission limits. Approaches to reducing emissions include improved in-cylinder combustion designs or fuel modifications, but these improvements have fallen short of meeting emissions limits. Other approaches involve the use of exhaust aftertreatment devices, which have achieved significant emissions reductions.
For lean burn internal combustion engines, such as diesel engines, the main pollutants of concern are oxides of nitrogen (NOx) and particulate matter (PM). The latter is composed of black smoke (soot), sulfates generated by sulfur in fuel, and components of unburned fuel and oil.
To reduce NOx, one approach is the use of NOx reduction devices, such as lean NOx traps (LNTs), lean NOx catalysts (LNCs), and selective catalytic reduction (SCR) catalysts. These devices typically use reductants, such as carbon monoxide and hydrogen for LNTs, extra diesel fuel for LNCs, and ammonia for SCR catalysts.
To reduce PM, one approach is the use of various types of diesel particulate filters (DPFs). Like NOx reduction devices, PM reduction devices often use a reductant, such as supplemental hydrocarbon fuel to produce an exotherm for regenerating the DPF.
NOx reduction devices and DPFs may be used alone or together, with either or both being used downstream of the engine, in the exhaust line.